In the manufacture of containers composed of aluminum and aluminum alloys, a drawing and forming operation is employed (commonly referred to as drawing and ironing). This operation results in the deposition of lubricants and forming oils on the surfaces of the aluminum containers. In addition, residual aluminum fines, i.e. small particles of aluminum, are deposited on both the interior and exterior surfaces. Ordinarily, the exterior surface of the container will have smaller quantities of aluminum fines than the interior surface since during the drawing and ironing step the exterior surface is not subjected to as much abrasion from the die as the interior surface.
Prior to any processing steps, such as conversion coating and sanitary lacquer deposition, the surfaces of the aluminum containers must be clean and water-break-free, i.e. free of contaminants that interfere with further processing and render the containers unacceptable for use.
Compositions and methods for the low temperature cleaning of aluminum surfaces are disclosed in U.S. Pat. No. 4,009,115 issued Feb. 22, 1977 to Robert Eric Binns, U.S. Pat. No. 4,116,853 issued Sept. 26, 1978 to Robert Eric Binns, U.S. Pat. No. 4,124,407 issued Nov. 7, 1978 to Robert Eric Binns, and U.S. Pat. No. 3,969,135 issued July 13, 1976 to Peter F. King, et al. These patents disclose cleaning compositions containing sulfuric acid, hydrofluoric acid or a fluoride salt, and a surfactant.
Compositions falling within the disclosures of these patents are commercially successful and are in fact used extensively in the cleaning of aluminum and aluminum alloy containers. Such commercial compositions typically utilize a combination of two nonionic surfactants to enhance the cleaning performance and minimize foaming.
One of the problems in utilizing the prior art acid cleaning compositions is caused by the build-up of lubricants and forming oils used in the drawing and forming of aluminum containers as these containers are treated with the cleaning solutions. The cleaning solutions must be replenished with fresh solutions from time to time to keep the oil level down. When the oil levels become excessive in the cleaning bath, containers cleaned in the bath exhibit significant water breaks after the cleaning solution is rinsed off. Water breaks are an indication that the surface of the aluminum is not clean and that oils or other foreign deposits are present. Such cans must be discarded or recleaned since they are not suitable for further processing as containers for beverages and other comestibles.
It has now been discovered that a particular anionic surfactant, i.e., an alkali metal 2-butoxyethoxyacetate, when used alone or in combination with one or more low foaming nonionic surfactants in the sulfur acid/hydrofluoric acid compositions of the prior art, has surprising advantages over known compositions containing the surfactants and surfactant combinations currently in use therein.
It has been found that the cleaning solutions of the present invention can tolerate relatively high concentrations of lubricants and forming oils without any water breaks occurring on the containers cleaned by such solutions. Hence, significant economies are realized since large numbers of containers can be processed before the operation must be shut down in order to replenish all or part of the cleaning solution.
Another significant advantage of the present cleaning compositions is the almost complete absence of foam in the cleaning bath and in the rinsing cycle following the cleaning step. Many of the acid cleaning compositions currently on the market exhibit problems with foam to a greater or lesser extent. Such compositions are mixtures of a high foaming nonionic surfactant to achieve good cleaning action with a low foaming anionic surfactant to try to contain the quantity of foam that would otherwise result. Foaming often results in overflow or dropping of the foam onto the floor of the facility containing the operation, resulting in slippery and unsafe conditions. Also, appearance of foam may lead to the operator's conclusion that the cleaning solution is not satisfactory due to the risks of foam overflow. Accordingly, the container cleaning operations must be shut down while the foam is skimmed off or the cleaning solution is replenished, leading to lost time and decreased flow-through of containers.
A further advantage of the present cleaning compositions is that the alkali metal 2-butoxyethoxyacetate can be utilized effectively in relatively small quantities, and this factor, combined with the already inexpensive cost of this surfactant (as the sodium salt) compared with the nonionic surfactants currently in use, results in great economic savings over the present commercial compositions. Significant cost savings are also realized even when a low foaming nonionic surfactant or a combination of such surfactants is also present in the cleaning composition of the invention, since the nonionic surfactant(s) is also present in relatively small quantities.
Typically, concentrates containing the sulfuric acid and the surfactant are prepared by the manufacturer and sold to container processing companies who make up cleaning solutions by diluting such concentrates with water and adding hydrofluoric acid to the solutions. Concentrates currently on the market tend to be rather highly colored, due to decomposition products formed from the action of concentrated sulfuric acid on the surfactants and/or from interactions between the surfactants and impurities in the commercial sulfuric acid which is commonly used in formulating the concentrates. Surprisingly, concentrates formed with an alkali metal 2-butoxyethoxyacetate as the sole surfactant present are colorless or only slightly yellow in color. Such concentrates are stable at very low temperatures, e.g. no precipitation occurs even at the temperature of a dry ice-acetone bath. Also, the concentrates are stable and do not discolor even when subjected to temperatures at 50.degree. C. for periods of three weeks or more. Most concentrates currently in use discolor even at room temperature, and precipitates form in some when containers of the concentrates are placed in dry ice-acetone baths. The great stability of the present compositions permits their shipment and storage under adverse temperature conditions without problems, a further significant economic advantage.
Another advantage of the present invention is the high level of cleanliness produced in the containers, particularly when a low foaming nonionic surfactant is also present, providing for unusually uniform conversion coating and lacquer deposition on the containers during their further processing.
Interestingly, when the only anionic surfactant disclosed in Binns' U.S. Pat. Nos. 4,009,115; 4,116,853; and 4,124,407, i.e. Tergitol Anionic 08 (sodium 2-ethyl hexyl sulfate), was tested in the acid cleaning compositions of these patents, water breaks occurred on the aluminum containers cleaned with such compositions after the addition of only small quantities of forming oils. Hence, the surprising advantages discovered with the anionic surfactant of the present invention appear to be unique and clearly are not advantages common to anionic surfactants in general.